The invention relates to towed marine cable control practice and more particularly but without limitation to the positioning of a towed seismic cable in a desired horizontal orientation by remote control.
Marine seismic exploration is often conducted by means of a marine cable containing a multiplicity of seismic sensors and known in the art as a streamer which is towed beneath the surface of the water by a vessel over an area to be seismic surveyed. Generally such surveys are conducted by towing the streamer along a predetermined line or track over the seafloor creating seismic disturbances in the water along that track by suitable means, and recording signals produced by the seismic sensors as a result. Cross-currents tend to drive the cable laterally off the track, a condition which degrades the quality of the seismic data collected. Thus it is desirable to be able to tow a seismic cable in a straight line along a selected heading despite the presence of cross-currents and the like which tend to drive segments or the entire cable laterally from the selected track.
Obstacles are sometimes encountered which force the towing vessel to break away from the track so as to avoid damaging the towed cable. As these cables are often two miles or more in length, a time consuming detour must be made to avoid the obstacle and to reposition the cable along the track. It is therefore also desirable to be able to controllably move selected sections of a marine cable laterally to the direction in which the cable being towed so as to, for example, steer the cable around the obstacles and to reposition the cable along track on the far side of the obstacle while avoiding a lengthy detour.
Lateral positioning of a towed cable comprises two basic aspects: determining the existing position of the cable and moving it to a desired position. Examining the latter aspect first, designs for remotely controllable lateral positioning devices for use with seismic and other towed marine cables are disclosed in U.S. Pat. Nos. 3,605,674 to Weese and 4,330,278 to Waters. Weese discloses several variations of a remotely controlled device for laterally or laterally and vertically positioning a streamer or other towed marine cable. Each device is mounted around the cable and, depending upon the embodiment, is provided with pairs of vertical or vertical and horizontal fins rotatable for directional control. Waters discloses a different embodiment of a depth and lateral positioning apparatus comprising vertically oriented hydrofoil supported from the sea surface by a float and in turn supporting a length of the towed cable by resilient connecting means. In Weese, an undefined signal is generated and transmitted in an undescribed fashion along wires within the cable being towed to appropriate sensing and actuating equipment within each lateral positioning device which accomplishes the rotation of the fins. Waters further discloses a control method and acoustic apparatus for automatically positioning a multiplicity of the lateral positioning devices along a straight line at a selected heading from the towing boat. The control method and apparatus described by Waters is unsuited for selectively positioning individual lateral positioning devices.
Several means for determining and monitoring the location of a towed cable are known. Weese and Waters both suggest locating and monitoring the position of the towed cable by means of sonar transponders positioned at selected points along the cable. The transponders can be used in a variety of ways with complementary equipment in the towing vessel to provide data from which the range and heading of each transponder with respect to the towing vessel can be calculated. Weese alternately suggests monitoring the location of the cable by horizontal ranging sonar, presumably on the towing vessel.
An entirely different method of locating a streamer or other marine cable with respect to a towing vessel is described in U.S. Pat. No. 3,953,827 to Le Moal et al. Le Moal et al discloses determining the locus of a towed cable by identifying the angles of tangents to the cable with respect to a fixed and known direction, such as magnetic north, at a plurality of known measuring points along the cable. The angular information is supplied by a suitable sensor located at each measuring point, preferably a magnetic compass. Means are also provided for coding and transmitting the measured angular values by means of electronic pulses to a central station. The position of each measuring point is determined by approximating that part of the towed cable located between the sensors to a circular arc, the length of which is known from the spacing of the sensors, while the angular value of the arc is determined from the differences between the angles formed by the tangent to the cable at the measuring points and the fixed and known direction. The positions of other known points along the cable are then determined by interpolation. Compared with the transducer locating methods described by Weese and Waters, the Le Moal et al method allows the more accurate determination of inflections in the cable between the measuring points. This information is of particular importance in seismic surveying where the precise location of the streamer sensors providing the seismic data is desired for correction purposes. The Le Moal et al method does not suffer from loss of sight of the cable which may be encountered by horizontal ranging sonar when the distal end of the towed cable is shielded under certain orientations by deflections of the cable lying in the line of sight of the sonar. Furthermore, the horizontal ranging sonar cable monitoring method does not identify the location of the lateral positioning devices along the cable nor does it provide cable positioning information in a form convenient for seismic data correcting. U.S. Pat. No. 4,068,208 to Rice, Jr. et al discloses yet another marine streamer position determining system which, however, is unsuited for cable monitoring use with lateral positioning devices.
U.S. patent application Ser. No. 885,916 filed Mar. 13, 1978, by Walter P. Neeley, which has been assigned to the assignee of this invention, discloses a marine seismic cable location system which utilizes cable tangent headings to determine the relative horizontal position of a cable with respect to a towing vessel and to produce a visual display of the towing vessel and cable for monitoring purposes.